The present invention relates to a ridge waveguide semiconductor laser diode, and more particularly, to a ridge waveguide semiconductor laser diode having a high output power and an excellent laser characteristic and capable of operating at a higher output power with excellent stability.
GaAs-based quantum-well laser diodes each having an InGaAs strained quantum-well active layer or layers, especially ridge waveguide semiconductor laser diodes, have been intensively researched and developed as feasible higher-output power semiconductor laser diodes or light sources in optical devices for use in wavelength division multiplexing (WDM) systems. With the development of WDM systems, it is desired to further increase the optical output power of the ridge waveguide semiconductor laser diode.
In addition, the GaAs based ridge waveguide semiconductor laser diode attracts attention as a light source for use in an erbium-doped fiber amplifier (EDFA).
Referring to FIG. 1, a typical GaAs-based quantum-well laser diode will be described as an example. A layer structure 10 of a conventional ridge waveguide GaAs laser diode includes, for instance, a buffer layer 14 having GaAs or AlGaAs based compound semiconductor layer, a lower cladding layer 16, a first optical confinement layer 18, a second optical confinement layer 20, a first strained quantum-well layer 22, a barrier layer 24, a second strained quantum-well layer 26, a third optical confinement layer 28, a fourth optical confinement layer 30, a first upper cladding layer 32, an etch stop layer 34, a second upper cladding layer 36, and a cap layer 38 consecutively formed on a n-GaAs substrate 12. The second upper cladding layer 36 and the cap layer 38 overlying the etch stop layer 34 have a mesa structure. The quantum-well layers 22,26, the barrier layer 24, and optical confinement layers 18,20,28,30 are oftentimes referred to as the active layers.
The chief factor for restricting the increase of the optical output power from the GaAs based ridge waveguide quantum-well laser diode (hereinafter referred to as simply GaAs quantum-well laser diode) is catastrophic optical damage (COD) which signifies that the optical facet of the laser diode is damaged instantaneously. When the optical output power increases to reach a specific value, the COD which is inherent in the GaAs based laser diode is generated to stop the function of the laser diode at that instant.
For preventing the generation of COD failure, for instance, a conventional wide mesa structure of about 4 xcexcm is adopted to decrease the optical density in the active layers: however, in case of the wide mesa structure, a so-called spatial-hole-burning phenomenon occurs wherein the optical gain of the laser diode is uneven in the direction parallel to the axis of the active layers arises to thereby tend to generate a beam steering phenomenon.
The beam steering phenomenon is known in the art and it means that the light beam moves in the direction parallel to the active layers (as viewed from the front facet of the laser), which causes the characteristic of the optical output with respect to injected current in the semiconductor laser diode to be non-linear by having one or more kinks in the characteristic, thereby deteriorating the laser characteristic significantly. The non-linearity means that the external differential quantum efficiency xe2x80x9cxcex7xe2x80x9d(xcex7=dLOUT/d(Ixe2x80x94ITH)) does not remain as a constant, wherein xe2x80x9cLOUTxe2x80x9d represents the optical output, xe2x80x9cIxe2x80x9d represents the injected current at the optical output of xe2x80x9cLOUTxe2x80x9d, and xe2x80x9cITHxe2x80x9d represents a threshold current. In a more extreme case, xe2x80x9cxcex7xe2x80x9d sometimes becomes almost 0. Accordingly, signal conversion cannot be effected from the injected current due to the deteriorated laser characteristic. This is particularly critical when the laser diode is coupled to an optical fiber.
For preventing the generation of the problem beam steering phenomenon in a high output power range, it is necessary to consider complicated and various effects, such as the waveguide mode control of the active layers, and thus the problem is not solved in the conventional GaAs quantum-well laser diode.
In addition, it is important to obtain an operational stability in the laser diode""s output at a higher output power levels, especially in the stability of the transverse and longitudinal modes of the diode when one uses a GaAs-based quantum-well laser diode as a light source for a WDM system.
As another application, a new optical module wherein a GaAs-based quantum-well laser diode and a fiber Bragg grating (FBG) for controlling the lasing wavelength and gain of the diode are integrated together is now being put to practical use as the pumping light source for erbium-doped fiber amplifiers (EDFAs). However, if a GaAs-based quantum-well laser diode having a lasing wavelength of 980 nm is used as a light source for an EDFA, the longitudinal mode operation is unstable when light returns from the FBG, whereby the optical output power fluctuates to cause another problem.
In the above description for the problems in the ridge waveguide semiconductor laser diodes, a GaAs-based quantum-well laser diode is exemplified. However, these problems are not peculiar to the GaAs-based quantum-well laser diode, but are common to general ridge waveguide semiconductor laser diodes.
It is therefore an object of the present invention to provide a ridge waveguide semiconductor laser diode having a linear characteristic between the optical output power and the injected current in a higher optical output range.
It is another object of the present invention to provide a ridge waveguide semiconductor laser diode which is capable of operating with excellent stability in both of its transverse and longitudinal modes.
It is still another object of the present invention to provide a ridge waveguide semiconductor laser diode which is capable of operating with a FBG in a stable manner to provide a stable light source for the EDFA.
The present invention provides a ridge waveguide semiconductor laser diode including a semiconductor substrate, at least one active layer overlying the semiconductor substrate, and at least one cladding layer overlaying the at least one active layer. The at least one cladding layer includes a ridge structure part having a length L along the direction of light propagation, a width S which is perpendicular to the length L and parallel to the surface of the at least one active layer, and a thickness T which is perpendicular to the width and the length. The at least one cladding layer further includes a remaining part which is a sub-layer disposed between the ridge structure part and the at least one active layer. The remaining part overlies at least the portions of the active layer(s) which are within a relatively short distance either side of the ridge structure part. This relatively short distance is typically around a value of 2xc2x7WH, where xe2x80x9cWHxe2x80x9d is the horizontal width of the laser spot emanating from the laser diode""s front facet, as measured at the front facet and as measured in the direction which is parallel with the plane of the active layers (as viewed from the front facet). However, in some preferred embodiments of the present invention, the remaining part of the cladding layer(s) overlies substantially the entire surface of the active layer(s).
For applications where the laser diode is not used with an FBG, or other type of wavelength selector, positioned in front of diode""s emitting facet, the remaining part has a thickness xe2x80x9cDxe2x80x9d in the range of:
Dxe2x89xa7xc2xdW, 
wherein xe2x80x9cWxe2x80x9d is the width of the laser spot emanating from the laser diode""s front facet, as measured at the front facet and as measured in the direction which is perpendicular to the plane of the active layers (as viewed from the front facet). The width is measured at the points of the spot where the light power level has a value which is {fraction (1/e2)} of the maximum power level at the center of the spot, with xe2x80x9cexe2x80x9d being the base of the natural logarithm (e=2.71828. . . ). In practice, and as described below in greater detail, the width of the spot can be readily measured from the vertical divergence angle in the laser""s far-field pattern (the latter of which is a projection of the laser spot onto a planar surface spaced at a distance from the laser diode""s front facet, and parallel thereto).
For applications where the laser diode is used with an FBG, or other type of wavelength selector, positioned in front of diode""s emitting facet, the remaining part has a thickness xe2x80x9cDxe2x80x9d within the range of:
1.10xc3x97W greater than Dxe2x89xa7xc2xdW, 
and more preferably within the range of:
1.05xc3x97W greater than Dxe2x89xa7xc2xdW, 
and most preferably within the range of:
xe2x80x83W greater than Dxe2x89xa7xc2xdW. 
In accordance with the semiconductor laser diode of the present invention, due to the value for xe2x80x9cDxe2x80x9d, the kink phenomenon in the characteristic of the optical output power with respect to injected current caused by a beam steering phenomenon can be improved, as a result of which an excellent linear characteristic can be obtained in a higher output power range, whereby the ridge waveguide semiconductor laser diode can operate with excellent stability in the transverse and longitudinal modes. In addition, the potential for COD failure is significantly reduced.
If an FBG is positioned at the front facet of a GaAs-based quantum well laser diode according to the present invention, the GaAs quantum well laser diode operates in a stable output power, and can be used to construct an EDFA having excellent operational characteristics.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.